will someone please explain to me the difference in power? Two engines, one being a 2 and one being a 4, each with the same HP rating.. it seems to me that because torque is a function of horsepower, that the torque should be the same.. however, you can turn a bigger prop with a four stroke.. hmm..

skukreja:

Torque and rpm do equal horsepower, that much is true. But generally speaking, the four stroke engine has its torque peak at a lower rpm than the two stroke. So for maximum power the four stroke is normally propped to run at a lower rpm, and this means a larger prop than found on the two stroke engine.

As I said, this is a generality, there are many exceptions finding the four stroke winding higher, and the two stroke engine chugging along like your old John Deere tractor.

Bill.

Sachin,


As Bill wrote, the RPM ranges are a generality.

The two-stroke has a power stroke on every revolution, while a four-stroke has one on every other revolution. So, even though the four-stroke has more effective scavenging and a higher combustion pressure, the two-stroke will produce more absolute torque.

If a two-stroke and a four-stroke are timed to produce their torque at the same RPM, the two-stroke will be capable of spinning a significantly larger prop.

Not so Dar, the torque figures are an average over a complete power cycle. Maximum negative torque occurs at about 20 degrees before the end of the compression stroke and maximum positive torque at about 20 degrees after TDC on the power stroke.

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Brian,


What is not so??? I fail to see the point.

At that 20 degrees ATDC in the power stroke, a four-stroke will have a greater torque value than a two-stroke of equal displacement.
But since the two-stroke enjoys this occurrence twice as many times (at the same RPM; other events are relatively minor in magnitude) as a four-stroke, the average positive torque is higher for a two-stroke, so it will be capable of spinning a larger prop at this RPM.

What is not correct in this paragraph?

OK, I see where you're coming from. You're going on the assumption that a 4 stroke can never equal the torque of a 2 stroke when both are optimised for a particular rev. I read what you said as meaning that if both a 4 stroke and a 2 stroke produced the same torque at the same revs then the 2 stroke could swing a bigger prop.

I think the difference is that because the 2 stroke is timed with ports that restrict the length of the stroke, the 2 stroke needs to rev higher to get the same torque as the 4 stroke.

The 4 stroke has an advantage with the use of valves, it can pull in more air and fuel, and will have a longer burn cycle than the 2 stroke. The 2 stroke can use it's higher revs to off set this advantage, but by doing so it needs a smaller prop.

MR Flyer57

Marshall,


From your explanation, one can understand that a four-stroke is always more powerful than a two-stroke engine.
Also, it is stated that a two-stroke engine will always need higher RPM levels, to achieve maximum torque.

Nothing can be further from the truth.
The design of any two-stroke engine places the ports, so they are timed to be effective at a certain target RPM range.

Even after it is built, it is possible to reverse-engineer an engine, changing its running characteristics completely.

Most larger two-strokes are timed to be at their best between 5,000 and 7,000 RPM.

Honda produced a four-stroke, inline six cylinder, 250 cc engine ([link=http://world.honda.com/goodwood/machines/rc166/]RC166[/link]), that was timed to be at its best power between 16,000 and 18,000 RPM...

Dropping the sleeve (and raising the head appropriately), would shorten the intake and the exhaust duration, improving the low RPM torque at the expense of high RPM performance. This would make a .40 spin a huge, 15x8 prop, at lower RPM...

Raising the sleeve (and dropping the head appropriately), would make a normal sport .40 get to 30,000 RPM, like a pylon engine...

These are examples and much more subtle changes are also possible. Changes require more changes; for higher RPM, the standard bearings must be replaced, for example.

There is always a trade-off. It would be quite costly to make an engine that is at its best everywhere.

Well there you go guys, why pay $500 or $1000 for a real pylon engine when all you have to do is shim up the liner a bit and shim down the head? All you need is $60 for a Magnum and next thing you know you're winning races

BTW, that's not reverse engineering, it's modifying. Reverse engineering is what the Chinese do with their OS clones. They take an engine known to work well, figure out why and copy it.

I realized my incorrect choice of words...
I will leave it.

As I wrote, modifications that are more than subtle will require more modification...

Raising the sleeve to support 30K, should require a gas passage larger than the original crankshaft's O/D...